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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>#if</span><span>#ifdef</span><span>#ifndef</span><span>#else</span><span>#elif</span><span>#elifdef</span><span>#elifndef</span><span>#endif</span></span></strong></div><div><span class="t-lines"><span></span><span></span><span></span><span></span><span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span><span></span></span></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="error.html" title="c/preprocessor/error"><span class="t-lines"><span>#error</span><span>#warning</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../../cpp/preprocessor/impl.html" title="cpp/preprocessor/impl"><span class="t-lines"><span>#pragma</span><span>_Pragma</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="line.html" title="c/preprocessor/line"><span class="t-lines"><span>#line</span></span></a></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by <code>#if</code>, <code>#else</code>, <code>#elif</code>, <code>#ifdef</code>, <code>#ifndef</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>, and <code>#endif</code> directives.
</p>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#if</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#ifdef</b></code> <span class="t-spar">identifier</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#ifndef</b></code> <span class="t-spar">identifier</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#elif</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#elifdef</b></code> <span class="t-spar">identifier</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-since-c23">(since C23)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#elifndef</b></code> <span class="t-spar">identifier</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-since-c23">(since C23)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#else</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#endif</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td> 
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The conditional preprocessing block starts with <code>#if</code>, <code>#ifdef</code> or <code>#ifndef</code> directive, then optionally includes any number of <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, or <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> directives, then optionally includes at most one <code>#else</code> directive and is terminated with <code>#endif</code> directive. Any inner conditional preprocessing blocks are processed separately.
</p><p>Each of <code>#if</code>, <code>#ifdef</code>, <code>#ifndef</code>, <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>, and <code>#else</code> directives control code block until first <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>, <code>#else</code>, <code>#endif</code> directive not belonging to any inner conditional preprocessing blocks. 
</p><p><code>#if</code>, <code>#ifdef</code> and <code>#ifndef</code> directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent <code>#else</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code>,</span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> and <code>#elif</code> directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent <code>#else</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code>,</span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> or <code>#elif</code> directive (if any) is processed. If the subsequent directive is <code>#else</code>, the code block controlled by the <code>#else</code> directive is unconditionally compiled. Otherwise, the <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, or <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> directive acts as if it was <code>#if</code> directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code>,</span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> and <code>#else</code> directives. The conditional preprocessing block is terminated by <code>#endif</code> directive.
</p>
<h3><span class="mw-headline" id="Conditional_evaluation">Conditional evaluation</span></h3>
<h4><span class="mw-headline" id=".23if.2C_.23elif"><code>#if, #elif</code></span></h4>
<p>The <span class="t-spar">expression</span> is a constant expression, using only <a href="../language/expressions.html#Constants_and_literals" title="c/language/expressions">constants</a> and identifiers, defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive. Any identifier, which is not literal, non defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive, evaluates to <i>0</i>. 
</p><p>The expression may contain unary operators in form <code>defined</code> <span class="t-spar">identifier</span> or  <code>defined (</code><span class="t-spar">identifier</span><code>)</code> which return <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">1</span></span></span> if the <span class="t-spar">identifier</span> was defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive and <i>0</i> otherwise. If the <span class="t-spar">expression</span> evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c23"><td>
<p>In context of a preprocessor directive, a <code>__has_c_attribute</code> expression detects whether a given attribute token is supported and its supported version. See <a href="../language/attributes.html#Attribute_testing" title="c/language/attributes">Attribute testing</a>.
</p>
</td>
<td><span class="t-mark-rev t-since-c23">(since C23)</span></td></tr>
</table>
<p>Note: Until DR 412, <code>#if cond1</code> ... <code>#elif cond2</code> is different from <code>#if cond1</code> ... <code>#else</code> followed by <code>#if cond3</code> because if <code>cond1</code> is true, the second <code>#if</code> is skipped and <code>cond3</code> does not need to be well-formed, while #elif's <code>cond2</code> must be a valid expression. As of DR 412, #elif that leads the skipped code block is also skipped.
</p>
<h4><span class="mw-headline" id="Combined_directives">Combined directives</span></h4>
<p>Checks if the identifier was <a href="replace.html" title="c/preprocessor/replace">defined as a macro name</a>. 
</p><p><code>#ifdef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#if defined</code> <span class="t-spar">identifier</span>.
</p><p><code>#ifndef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#if !defined</code> <span class="t-spar">identifier</span>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c23"><td>
<p><code>#elifdef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#elif defined</code> <span class="t-spar">identifier</span>.
</p><p><code>#elifndef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#elif !defined</code> <span class="t-spar">identifier</span>.
</p>
</td>
<td><span class="t-mark-rev t-since-c23">(since C23)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>While <code>#elifdef</code> and <code>#elifndef</code> directives target C23, implementations may backport them to the older language modes as conforming extensions.
</p>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#define ABCD 2</span>
<span class="co2">#include &lt;stdio.h&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
 
<span class="co2">#ifdef ABCD</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: no<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#ifndef ABCD</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no1<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#elif ABCD == 2</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no2<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#if !defined(DCBA) &amp;&amp; (ABCD &lt; 2*4-3)</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"3: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co1">// C23 directives #elifdef/#elifndef</span>
<span class="co2">#ifdef CPU</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"4: no1<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#elifdef GPU</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"4: no2<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#elifndef RAM</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"4: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// selected in C23 mode, may be selected in pre-C23 mode</span>
<span class="co2">#else</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"4: no3<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// may be selected in pre-C23 mode</span>
<span class="co2">#endif</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1: yes
2: yes
3: yes
4: yes</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_412">DR 412</a>
</td>
<td> C89
</td>
<td> failed <code>#elif</code>'s expression was required to be valid
</td>
<td> failed <code>#elif</code> is skipped
</td></tr></table>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-17">
<ul><li> C17 standard (ISO/IEC 9899:2018): 
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 118-119)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 162-164)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 147-149)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 3.8.1 Conditional inclusion 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

<!-- 
NewPP limit report
Preprocessor visited node count: 2744/1000000
Preprocessor generated node count: 7960/1000000
Post‐expand include size: 52198/2097152 bytes
Template argument size: 12531/2097152 bytes
Highest expansion depth: 16/40
Expensive parser function count: 0/100
-->

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